a) include the rendering module as shown here for Padrino 0.9.26 (which simply makes the render method more powerful)
b) use an explicit engine as shown partial("foo", :engine => :erb) in edge padrino after this commit right now

Note that b only works after changes I just made in Padrino so you must work off git because in edge padrino, I also added another way to get partials working without the need for augmenting rendering: